Best way to get green nickel oxide into solution ?

Quote: Originally posted by Nemo_Tenetur

I thought perchloric is one of the strongest acids and nickel perchlorate is also very well soluble? I also found a statement that nickel oxide is soluble im ammonia water, maybe i should try this even if i´m not convinced.

Quote: Originally posted by Nemo_Tenetur

Thank you for this information. The first link doesn´t work

Quote: Originally posted by Nemo_Tenetur

The German Wikipedia states that nickel oxide is soluble in ammonia water and refers to "Georg Bauer, handbook of preparative inorganic chemistry"

Quote: Originally posted by clearly_not_atara

I would definitely prefer hydrochloric acid, since chloride is a weak ligand for nickel. Perchlorate on the other hand is a highly non-coordinating anion.

Quote: Originally posted by bnull

You don't need the nickel salts right away, do you? Put a little oxide into a vial, add some acid and leave the vial alone for a couple of days or a week. Do that with the acids you have (and ammonia too) and see later what dissolved the oxide the most. Give them time and go do something else. I remember some iron oxide sold as pigment that refused to dissolve in cold, hot, and boiling hydrochloric acid. I gave up and forgot the matter. Some time later, while digging through my stuff, I saw that a good part of the oxide had dissolved.

Update

Just for your information, a prolonged refluxing with about 35 % perchloric acid for 48 - 72 hours dissolved most of the nickel oxide.

After cooling to room temperature, a vacuum filtration with a buchner funnel yielded a heavy somewhat viscous clear dark green solution.

I´ve done two batches. First batch with about 48 hours refluxing yielded a dark green solution with a measured density of 1.49 gram per milliliter at seven degree centigrade. It didnt´t crystallize after three days at 7 degree centigrade. So the whole batch was evaporated to dryness at about 40 - 50 degree centigrade in a very large open pan. Yield about 60% of the weight from the heavy solution.

The second batch was refluxed for about 72 hours, the nickel oxide went almost completely into solution, only a few percent residue. I´ve used an almost stoichometric quantity of perchloric acid (no excess, it´s much more expensive and valuable than nickel oxide) and I guess it was almost completely used.

The measured density of the dark green solution of this batch was 1.53 gram per milliliter. It´s still evaporating and I expect a slightly better yield, maybe somewhat above 60% (calculated by the weight of the solution by evaporation to dryness). The total conversion rate (nickel oxide to nickel perchlorate) is probably about 95% .

Thank you @Axt for the perchloric acid refluxing suggestion!

@woelen I think there´re some typos in the corresponding sciencemadness wiki article, see attached screenshot.

"Nickel(II) nitrate is the chemical compound with the formula Ni(ClO4)2."

I doubt that the solubility data is correct. From my experience, a nickel perchlorate solution with a concentration of more than 60% by weight is possible even at 7 degree centigrade. So even if the solubility is calculated as the anhydrous salt , it´s too low.

Thank you for explanation. That means, even a considerable residue of perchloric acid is not detectable by smell.

By the way, do you know if the hexammine complex of nickel perchlorate is labile? I´ve dissolved the residue of my nickel perchlorate in the bowl and glassware with several milliliter distilled water and added excess 25 % ammonia water. Immediately a deep blue complex precipitated out and was filtered yesterday in the evening, followed by washing with distilled water (no more ammonia smell was detectable) and I let it stand outside in the cold.

Today the precipitate wasn´t dry (that´s no surprise as the temperature outside was somewhat below zero degree centigrade), but it was partially discolored back to greenish.

See attached jpeg.



I also noticed some weak ammonia smell.

Is it possible that it loses some ammonia? I have no other explanation for this oberservation. I guess (pure speculation!) that the ammonia loss was only partially, not completely, because otherwise the extremely soluble nickel perchlorate probably wouldn´t precipitate in such a wet environment.